I can get you started on a possible script implementation.
If you look at Game_Battler, under the various stats (let's pick Strength for example), you'll see
def str
n = [[base_str + @str_plus, 1].max, 999].min
for i in @states
n *= $data_states[i].str_rate / 100.0 # <========= This line right here
end
n = [[Integer(n), 1].max, 999].min
return n
end
You can refer to the Help file for RPG::State for the various parameters you can access.
So you might be thinking "Cool, I can just manually set this value to whatever I want by doing"
$data_states[13].str_rate = 500
Which, yes, you can, but you must specifically make this call at some point after Scene_Title loads, as seen in its
main method.
It's entirely up to you how you want to make these calls. You can alias Scene_Title#main and make your modifications at the end, like so:
class Scene_Title
alias define_custom_state_params main
def main
define_custom_state_params
$data_states[13].str_rate = 500
end
end
Now, whenever you transition to a new scene after Scene_Title, you override the database values with your own custom values.